The present invention relates to the technical field of rotary magnetic sensors of the type including an encoder component moving near a detector cell and suitable for locating at least one angular position, in the broadest sense.
The object of the invention finds a particularly advantageous application in the automotive field where such a sensor may be used, for example within the framework of ignition functions.
In the preferred field above, it is known how to apply a suitable magnetic sensor for measuring the change in the strength of a magnetic field, when an encoder provided with a series of components generating a variable magnetic field, moves past one or more measuring or detector cells. Each detector cell, such as for example a Hall effect or magneto-resistive probe, delivers a periodic electrical signal corresponding to the change in the strength of the magnetic field generated by the components. Each detector cell is associated with a level comparator with hysteresis, such as a Schmitt trigger, in order to obtain clear transitions of the output voltage for distinct values of the magnetic field, according to whether it varies by increasing or decreasing its value.
In order to form a sensor for detecting a velocity of rotation, it is known how to make an encoder provided with components generating a variable magnetic field, regularly laid out along a circumference.
According to a first embodiment, the generating components consist of components which perturb a magnetic field created by a fixed magnet placed near such perturbing components. For example, such perturbing components are formed by teeth provided in a ferromagnetic ring.
According to a second embodiment, the components generating a variable magnetic field are formed by magnetic poles, regularly spaced out according to a given pitch. Such an encoder thus appears as a multipolar magnetic ring.
In order to be able to determine at least one position, for example corresponding to the top dead centre for igniting a cylinder, it is known how to produce a singularity on the magnetic encoder commonly called a singular or irregular pole. Thus, for example, it is known how to suppress two teeth on the ferromagnetic ring. In the alternative embodiment applying an encoder made as a multipolar magnetic ring, suppressing several magnetic poles by leaving a remaining empty space, or replacing one or several poles of a given sign with one or several poles of an opposite sign, may be contemplated. A given magnetization pole is thereby achieved, which between both of its adjacent poles with an opposite sign, has a gap pitch different from the gap pitch of the other poles.
Under certain conditions of use, there also occurs the need of making regular magnetic poles with different gap pitches in order to obtain a signal with a cyclic ratio different from 0.5. Generally, it should be noted that the making of an encoder with magnetic poles having different gap pitches, i.e., with magnets of different sizes, leads to magnetic instability causing a lack of accuracy in the detection of the positions.